Avoid SHM inference all-reduce path for world_size==1

Signed-off-by: Masahiro Tanaka <mtanaka@anyscale.com>

Author: tohtana

deepspeed/comm/torch.py

@@ -167,7 +167,11 @@ def all_reduce(self, tensor, op=torch.distributed.ReduceOp.SUM, group=None, asyn
         return torch.distributed.all_reduce(tensor=tensor, op=op, group=group, async_op=async_op)
 
     def inference_all_reduce(self, tensor, op, group=None):
-        if not hasattr(torch.ops, 'deepspeed') or not hasattr(torch.ops.deepspeed, 'inference_all_reduce_'):
+        use_ds_op = hasattr(torch.ops, 'deepspeed') and hasattr(torch.ops.deepspeed, 'inference_all_reduce_')
+        world_size = torch.distributed.get_world_size(group=group)
+        if world_size <= 1:
+            return tensor
+        if not use_ds_op:
             op = self._reduce_op(op)
             return torch.distributed.all_reduce(tensor=tensor, op=op, group=group, async_op=False)
         else: